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Abstract:

A zoom lens system includes a negative first lens group and a positive
second lens group, in that order from the object side, wherein upon
zooming from the short focal length extremity to the long focal length
extremity, the first lens group and the second lens group move in the
optical axis direction while the distance therebetween mutually
decreases. The first lens group includes a negative first sub-lens group
and a positive second sub-lens group, in that order from the object side,
wherein the second sub-lens group constitutes a focusing lens group that
is moved in the optical axis direction during a focusing operation.

Claims:

1. A zoom lens system comprising a negative first lens group and a
positive second lens group, in that order from the object side, wherein
upon zooming from the short focal length extremity to the long focal
length extremity, said first lens group and said second lens group move
in the optical axis direction while the distance therebetween mutually
decreases, wherein said first lens group includes a negative first
sub-lens group and a positive second sub-lens group, in that order from
the object side, wherein said second sub-lens group constitutes a
focusing lens group that is moved in the optical axis direction during a
focusing operation.

2. The zoom lens system according to claim 1, wherein said second
sub-lens group comprises a positive single lens element.

3. The zoom lens system according to claim 2, wherein the following
condition (1) is satisfied: -1<SF<0 (1), wherein
SF=(br1-br2)/(br1+br2), br1 designates the radius of curvature of the
surface on the object side of said positive single lens element of said
second sub-lens group, and br2 designates the radius of curvature of the
surface on the image side of said positive single lens element of said
second sub-lens group.

4. The zoom lens system according to claim 1, wherein the following
condition (2) is satisfied: -5.0<f1b/f1a<-3.5 (2), wherein f1b
designates the focal length of said second sub-lens group, and f1a
designates the focal length of said first sub-lens group.

5. The zoom lens system according to claim 1, wherein said first sub-lens
group comprises three negative lens elements which each has a concave
surface on the image side.

6. The zoom lens system according to claim 5, wherein the following
condition (3) is satisfied: -3.5<fL1/(fL2*fL3)1/2<-1.0 (3),
wherein fL1 designates the focal length of the first negative lens
element that is provided within said first sub-lens group, in that order
from the object side, fL2 designates the focal length of the second
negative lens element that is provided within said first sub-lens group,
in that order from the object side, and fL3 designates the focal length
of the third negative lens element that is provided within said first
sub-lens group, in that order from the object side.

7. The zoom lens system according to claim 5, wherein an aspherical
surface is provided on at least one of said three negative lens elements
of said first sub-lens group.

8. The zoom lens system according to claim 7, wherein the negative lens
element that is provided closest to the object side within said first
sub-lens group is a hybrid comprising a glass lens element having a
compound resin layer bonded to the image side thereof.

9. The zoom lens system according to claim 7, wherein an aspherical
surface is provided on the second negative lens element that is provided
within said first sub-lens group, in that order from the object side.

10. The zoom lens system according to claim 1, wherein at least one
aspherical-surfaced lens element is provided in each of said first lens
group and said second lens group.

11. The zoom lens system according to claim 1, wherein said second lens
group comprises at least three positive lens elements.

12. The zoom lens system according to claim 1, wherein the air-distance
between said first sub-lens group and said second sub-lens group remains
unchanged during zooming from the short focal length extremity to the
long focal length extremity.

13. The zoom lens system according to claim 1, wherein a diaphragm is
provided between said first lens group and said second lens group.

14. An optical instrument comprising an image sensor that electronically
converts an image that is formed through the zoom lens system according
to claim 1 into a signal.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a zoom lens system that is
suitable for use in an optical instrument such as a digital camera, etc.

[0003] 2. Description of Related Art

[0004] In recent years there has been an increasing need for a zoom lens
system in an optical instrument such as a digital camera, etc., to be
more compact (miniaturized) and to have a higher optical quality. There
is also a very strong demand for miniaturization of the focusing
mechanism system and for a rapid focusing operation.

[0005] Zoom lens systems configured of a negative first lens group and a
positive second lens group, in that order from the object side, are known
in the art. A so-called front focusing method, in which the entire first
lens group is moved along the optical axis to carry out a focusing
operation, is a typical focusing method that is used in such a type of
zoom lens system.

[0006] However, in such a front focusing method, if the weight of the
first lens group, which constitutes a focusing lens group, is large (if
the number of lens elements in the first lens group is large), the
motor/actuator that constitutes the focusing mechanism system is also
enlarged. Accordingly, the diameter of the lens barrel (which includes
the zoom lens system of the present invention and the motor/actuator) is
enlarged, thereby enlarging the entire zoom lens system.

[0007] Japanese Unexamined Patent Publication No. 2004-93593 discloses a
zoom lens system configured of a negative first lens group and a positive
second lens group, in that order from the object side, in which the two
lens elements provided on the image side within the first lens group are
used as a focusing lens group.

[0008] However, the burden on the focusing mechanism system such as the
motor/actuator still remains great, so that such a focusing mechanism
system cannot adequately cope with rapid focusing operations.

SUMMARY OF THE INVENTION

[0009] The present invention, in view of the above-discussed problems,
provides a zoom lens system which is compact (miniaturized), has a
superior optical quality, achieves miniaturization of the focusing
mechanism system, and achieves a rapid focusing operation; the present
invention also provides an optical instrument which uses such a zoom lens
system.

[0010] According to an aspect of the present invention, a zoom lens system
is provided, including a negative first lens group and a positive second
lens group, in that order from the object side, wherein upon zooming from
the short focal length extremity to the long focal length extremity, the
first lens group and the second lens group move in the optical axis
direction while the distance therebetween mutually decreases. The first
lens group includes a negative first sub-lens group and a positive second
sub-lens group, in that order from the object side, wherein the second
sub-lens group constitutes a focusing lens group that is moved in the
optical axis direction during a focusing operation.

[0011] It is desirable for the second sub-lens group to include a positive
single lens element.

[0012] It is desirable for the following condition (1) to be satisfied:

-1<SF<0 (1),

wherein

[0013] SF=(br1-br2)/(br1+br2), br1 designates the radius of curvature of
the surface on the object side of the positive single lens element of the
second sub-lens group, and br2 designates the radius of curvature of the
surface on the image side of the positive single lens element of the
second sub-lens group.

[0014] It is further desirable for the following condition (1') to be
satisfied:

-0.85<SF<-0.40 (1').

[0015] It is desirable for the following condition (2) to be satisfied:

-5.0<f1b/f1a<-3.5 (2),

wherein f1b designates the focal length of the second sub-lens group, and
f1a designates the focal length of the first sub-lens group.

[0016] It is desirable for the first sub-lens group to include three
negative lens elements which each has a concave surface on the image
side. For example, a negative lens element having a concave surface on
the image side can refer to a negative meniscus lens element having a
concave surface on the image side or a biconcave negative lens element.

[0017] It is desirable for the following condition (3) to be satisfied:

-3.5<fL1/(fL2*fL3)1/2<-1.0 (3),

wherein fL1 designates the focal length of the first negative lens
element that is provided within the first sub-lens group, in that order
from the object side, fL2 designates the focal length of the second
negative lens element that is provided within the first sub-lens group,
in that order from the object side, and fL3 designates the focal length
of the third negative lens element that is provided within the first
sub-lens group, in that order from the object side.

[0018] It is desirable for an aspherical surface to be provided on at
least one of the three negative lens elements of the first sub-lens
group.

[0019] It is desirable for the negative lens element that is provided
closest to the object side within the first sub-lens group to be a hybrid
including a glass lens element having a compound resin layer bonded to
the image side thereof.

[0020] Alternatively, it is desirable for an aspherical surface to be
provided on the second negative lens element that is provided within the
first sub-lens group, in that order from the object side.

[0021] It is desirable for at least one aspherical-surfaced lens element
to be provided in each of the first lens group and the second lens group.

[0022] It is desirable for the second lens group to include at least three
positive lens elements.

[0023] It is desirable for the air-distance between the first sub-lens
group and the second sub-lens group to remain unchanged during zooming
from the short focal length extremity to the long focal length extremity
(in which the first sub-lens group and the second sub-lens group
integrally move in the optical axis direction during zooming).

[0024] It is desirable for a diaphragm to be provided between the first
lens group and the second lens group.

[0025] In an embodiment, an optical instrument is provided, including an
image sensor that electronically converts an image that is formed through
the above-described zoom lens system.

[0026] According to the present invention, a zoom lens system is achieved
which is compact (miniaturized), has a superior optical quality, achieves
miniaturization of the focusing mechanism system, and achieves a rapid
focusing operation; the present invention also provides an optical
instrument which uses such a zoom lens system.

[0027] The present disclosure relates to subject matter contained in
Japanese Patent Application No. 2011-120494 (filed on May 30, 2011) which
is expressly incorporated herein in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention will be discussed below in detail with
reference to the accompanying drawings, in which:

[0029]FIG. 1 shows a lens arrangement of a first numerical embodiment of
a zoom lens system, according to the present invention, at the long focal
length extremity when focused on an object at infinity;

[0030] FIGS. 2A, 2B, 2C and 2D show various aberrations that occurred in
the lens arrangement shown in FIG. 1;

[0031] FIGS. 3A, 3B, 3C and 3D show lateral aberrations that occurred in
the lens arrangement shown in FIG. 1;

[0032]FIG. 4 shows a lens arrangement of the first numerical embodiment
of the zoom lens system, according to the present invention, at the short
focal length extremity when focused on an object at infinity;

[0033] FIGS. 5A, 5B, 5C and 5D show various aberrations that occurred in
the lens arrangement shown in FIG. 4;

[0034] FIGS. 6A, 6B, 6C and 6D show lateral aberrations that occurred in
the lens arrangement shown in FIG. 4;

[0035]FIG. 7 shows a lens arrangement of a second numerical embodiment of
a zoom lens system, according to the present invention, at the long focal
length extremity when focused on an object at infinity;

[0036] FIGS. 8A, 8B, 8C and 8D show various aberrations that occurred in
the lens arrangement shown in FIG. 7;

[0037] FIGS. 9A, 9B, 9C and 9D show lateral aberrations that occurred in
the lens arrangement shown in FIG. 7;

[0038] FIG. 10 shows a lens arrangement of the second numerical embodiment
of the zoom lens system, according to the present invention, at the short
focal length extremity when focused on an object at infinity;

[0039] FIGS. 11A, 11B, 11C and 11D show various aberrations that occurred
in the lens arrangement shown in FIG. 10;

[0040] FIGS. 12A, 12B, 12C and 12D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 10;

[0041] FIG. 13 shows a lens arrangement of a third numerical embodiment of
a zoom lens system, according to the present invention, at the long focal
length extremity when focused on an object at infinity;

[0042] FIGS. 14A, 14B, 14C and 14D show various aberrations that occurred
in the lens arrangement shown in FIG. 13;

[0043] FIGS. 15A, 15B, 15C and 15D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 13;

[0044] FIG. 16 shows a lens arrangement of the third numerical embodiment
of the zoom lens system, according to the present invention, at the short
focal length extremity when focused on an object at infinity;

[0045] FIGS. 17A, 17B, 17C and 17D show various aberrations that occurred
in the lens arrangement shown in FIG. 16;

[0046] FIGS. 18A, 18B, 18C and 18D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 16;

[0047] FIG. 19 shows a lens arrangement of a fourth numerical embodiment
of a zoom lens system, according to the present invention, at the long
focal length extremity when focused on an object at infinity;

[0048] FIGS. 20A, 20B, 20C and 20D show various aberrations that occurred
in the lens arrangement shown in FIG. 19;

[0049] FIGS. 21A, 21B, 21C and 21D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 19;

[0050] FIG. 22 shows a lens arrangement of the fourth numerical embodiment
of the zoom lens system, according to the present invention, at the short
focal length extremity when focused on an object at infinity;

[0051] FIGS. 23A, 23B, 23C and 23D show various aberrations that occurred
in the lens arrangement shown in FIG. 22;

[0052] FIGS. 24A, 24B, 24C and 24D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 22;

[0053] FIG. 25 shows a lens arrangement of a fifth numerical embodiment of
a zoom lens system, according to the present invention, at the long focal
length extremity when focused on an object at infinity;

[0054] FIGS. 26A, 26B, 26C and 26D show various aberrations that occurred
in the lens arrangement shown in FIG. 25;

[0055] FIGS. 27A, 27B, 27C and 27D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 25;

[0056]FIG. 28 shows a lens arrangement of the fifth numerical embodiment
of the zoom lens system, according to the present invention, at the short
focal length extremity when focused on an object at infinity;

[0057] FIGS. 29A, 29B, 29C and 29D show various aberrations that occurred
in the lens arrangement shown in FIG. 28;

[0058] FIGS. 30A, 30B, 30C and 30D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 28;

[0059] FIG. 31 shows a lens arrangement of a sixth numerical embodiment of
a zoom lens system, according to the present invention, at the long focal
length extremity when focused on an object at infinity;

[0060] FIGS. 32A, 32B, 32C and 32D show various aberrations that occurred
in the lens arrangement shown in FIG. 31;

[0061] FIGS. 33A, 33B, 33C and 33D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 31;

[0062]FIG. 34 shows a lens arrangement of the sixth numerical embodiment
of the zoom lens system, according to the present invention, at the short
focal length extremity when focused on an object at infinity;

[0063] FIGS. 35A, 35B, 35C and 35D show various aberrations that occurred
in the lens arrangement shown in FIG. 34;

[0064] FIGS. 36A, 36B, 36C and 36D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 34;

[0065]FIG. 37 shows a lens arrangement of a seventh numerical embodiment
of a zoom lens system, according to the present invention, at the long
focal length extremity when focused on an object at infinity;

[0066] FIGS. 38A, 38B, 38C and 38D show various aberrations that occurred
in the lens arrangement shown in FIG. 37;

[0067] FIGS. 39A, 39B, 39C and 39D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 37;

[0068]FIG. 40 shows a lens arrangement of the seventh numerical
embodiment of the zoom lens system, according to the present invention,
at the short focal length extremity when focused on an object at
infinity;

[0069] FIGS. 41A, 41B, 41C and 41D show various aberrations that occurred
in the lens arrangement shown in FIG. 40;

[0070] FIGS. 42A, 42B, 42C and 42D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 40;

[0071]FIG. 43 shows a lens arrangement of an eighth numerical embodiment
of a zoom lens system, according to the present invention, at the long
focal length extremity when focused on an object at infinity;

[0072] FIGS. 44A, 44B, 44C and 44D show various aberrations that occurred
in the lens arrangement shown in FIG. 43;

[0073] FIGS. 45A, 45B, 45C and 45D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 43;

[0074]FIG. 46 shows a lens arrangement of the eighth numerical embodiment
of the zoom lens system, according to the present invention, at the short
focal length extremity when focused on an object at infinity;

[0075] FIGS. 47A, 47B, 47C and 47D show various aberrations that occurred
in the lens arrangement shown in FIG. 46;

[0076] FIGS. 48A, 48B, 48C and 48D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 46;

[0077]FIG. 49 shows a lens arrangement of a ninth numerical embodiment of
a zoom lens system, according to the present invention, at the long focal
length extremity when focused on an object at infinity;

[0078] FIGS. 50A, 50B, 50C and 50D show various aberrations that occurred
in the lens arrangement shown in FIG. 49;

[0079] FIGS. 51A, 51B, 51C and 51D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 49;

[0080] FIG. 52 shows a lens arrangement of the ninth numerical embodiment
of the zoom lens system, according to the present invention, at the short
focal length extremity when focused on an object at infinity;

[0081] FIGS. 53A, 53B, 53C and 53D show various aberrations that occurred
in the lens arrangement shown in FIG. 52;

[0082] FIGS. 54A, 54B, 54C and 54D show lateral aberrations that occurred
in the lens arrangement shown in FIG. 52; and

[0083]FIG. 55 shows a zoom path of the zoom lens system according to the
present invention.

DESCRIPTION OF THE EMBODIMENTS

[0084] The zoom lens system of the illustrated embodiments, as shown in
the zoom path of FIG. 55, is configured of a negative first lens group
G1, and a positive second lens group G2, in that order from the object
side. The first lens group G1 is configured of a negative first sub-lens
group G1a and a positive second sub-lens group G1b, in that order from
the object side. A diaphragm S which is provided between the second
sub-lens group G1b (of the first lens group G1) and the second lens group
G2 moves integrally with the second lens group G2 along the optical axis
direction. `I` designates the imaging plane.

[0085] In the zoom lens system of the present invention, upon zooming from
the short focal length extremity (W) to the long focal length extremity
(T), the distance between the first lens group G1 and the second lens
group G2 decreases. Upon zooming from the short focal length extremity
(W) to the long focal length extremity (T), the air-distance between the
first sub-lens group G1a and the second sub-lens group G1b does not
change (the first sub-lens group G1a and the second sub-lens group G1b
integrally move in the optical axis direction).

[0086] More specifically, upon zooming from the short focal length
extremity (W) to the long focal length extremity (T), the first lens
group G1 (the first sub-lens group G1a and the second sub-lens group G1b)
first moves toward the image side and thereafter moves by a slight amount
toward the object side (thereby moving toward the image side as a whole),
and the second lens group G2 moves monotonically toward the object side.

[0087] In each of the first through ninth numerical embodiments, the first
sub-lens group G1a is configured of three negative lens elements
(negative lens elements each having a concave surface on the image side)
11, 12 and 13, in that order from the object side. In each of the first
through sixth, eighth and ninth numerical embodiments, the negative lens
element 11 that is provided closest to the object side is formed as a
hybrid lens configured of a glass lens element having an aspherical
layer, formed by a compound resin material, bonded to the image side
thereof; in the seventh numerical embodiment, the negative lens element
11 is a spherical lens element (i.e., is not a hybrid lens). In each of
the first through sixth, eighth and ninth numerical embodiments, the
second negative lens element 12 from the object side is a spherical lens
element; in the seventh numerical embodiment, the second negative lens
element 12 from the object side has an aspherical surface on each side
thereof.

[0088] In each of the first through ninth numerical embodiments, the
second sub-lens group G1b is configured of a positive single lens element
14. The positive single lens element (second sub-lens group G1b) 14
constitutes a focusing lens group which is moved in the optical axis
direction during a focusing operation. Namely, when focusing on an object
at infinity through to an object at a finite distance, focusing is
carried out by moving the positive single lens element 14 toward the
image side.

[0089] In each of the first through sixth and eighth numerical
embodiments, the second lens group G2 is configured of a positive lens
element 21, a cemented lens formed from a positive lens element 22 and a
negative lens element 23; and a positive lens element 24, in that order
from the object side. Each of the positive lens elements 21 and 24 has an
aspherical surface on each side thereof.

[0090] In each of the seventh and ninth numerical embodiments, the second
lens group G2 is configured of a positive lens element 21', a positive
lens element 22', a cemented lens formed from a positive lens element 23'
and a negative lens element 24'; and a positive lens element 25', in that
order from the object side. Each of the positive lens elements 22' and
25' has an aspherical surface on each side thereof.

[0091] In the illustrated embodiments, in order to achieve a negative
refractive power in the zoom lens system while suppressing occurrence of
distortion, the first lens group G1 is configured of the first sub-lens
group G1a, which is configured of the three negative lens elements
(negative lens elements each having a concave surface on the image side)
11, 12 and 13; and the second sub-lens group G1b which is configured of
the positive single lens element 14.

[0092] In order to suppress distortion, it is effective to provide a
positive lens element (having a convex surface on the object side) at a
location that is closest to the object side within the first lens group.
However, if such a positive lens element (having a convex surface on the
object side) is provided at a location that is closest to the object
side, the maximum diameter of the first lens group becomes too large,
thereby increasing the overall size of the entire zoom lens system.

[0093] Therefore, in the illustrated embodiments, by arranging the first
sub-lens group G1a so as to be configured of the three negative lens
elements 11, 12 and 13, and by including a lens element
(aspherical-surface lens element) that has at least one aspherical
surface within the first sub-lens group G1a, enlargement of the first
lens group G1 can be prevented, and occurrence of distortion can be
successfully suppressed.

[0094] From a viewpoint of cost, it is advantageous for the
aspherical-surface lens element within the first sub-lens group G1a to be
located closest to the image side so as to have the smallest diameter
(i.e., the negative lens element 13); however, there is, nevertheless,
the disadvantage of aberration correction being insufficient since the
lens diameter (of the aspherical-surface lens element) is small.

[0095] To solve this problem, the illustrated embodiments achieve
favorable aberration correction by configuring the negative lens element
11 that is provided closest to the object side within the first sub-lens
group G1a or the second negative lens element 12 from the object side
within the first sub-lens group G1a as the aspherical-surface lens
element. In the case where the negative lens element 11 that is provided
closest to the object side within the first sub-lens group G1a is
configured as the aspherical-surface lens element, in view of the
manufacturing costs, it is desirable to form the negative lens element 11
as a hybrid lens configured of a glass lens element having an aspherical
layer, formed by a compound resin material, bonded to the image side
thereof. If the aspherical surface (of the negative lens element 11) that
is included within the first sub-lens group G1a is formed such that the
negative refractive power thereof increasingly weakens (the positive
refractive power increasingly strengthens) from the optical axis toward
the outer periphery compared to the paraxial spherical surface thereof,
positive distortion occurs at this aspherical surface to thereby
favorably correct the negative distortion that prominently occurs at the
first lens group G1.

[0096] The positive single lens element 14 of the second sub-lens group
G1b constitutes a focusing lens group that is moved in the optical axis
direction during a focusing operation, and also functions to prevent
fluctuation in distortion, spherical aberration and coma during a
focusing operation. By arranging the focusing lens group so as to be
configured of the positive single lens element 14, the weight of the
focusing lens group can be reduced, the motor/actuator that constitutes
the focusing mechanism system can be miniaturized. Accordingly, the
maximum diameter of the lens barrel (which includes the zoom lens system
of the present invention) can be reduced and the entire zoom lens system
can also be miniaturized. Moreover, a rapid focusing operation can also
be achieved.

[0097] If the shape of the positive single lens element 14 is formed as a
meniscus shape having a convex surface on the object side, abaxial
astigmatism at the short focal length extremity can be favorably
corrected.

[0098] In the illustrated embodiments, by including at least one negative
lens element (the negative lens element 23 or the negative lens element
24'), which generates negative spherical aberration, within the second
lens group G2, spherical aberrations that occur over the entire zoom lens
system can be favorably corrected while retaining a minimal influence on
the abaxial aberration. Furthermore, by including at least three positive
lens elements (the positive lens elements 21, 22 and 24, or the positive
lens elements 21', 22', 23' and 25') within the second lens group G2,
occurrence of spherical aberration and coma can be suppressed.
Furthermore, by bonding the negative lens element provided within the
second lens group G2 with one positive lens element (i.e., bonding the
positive lens element 22 with the negative lens element 23, or bonding
the positive lens element 23' with the negative lens element 24'),
high-order spherical aberrations can also be favorably corrected.

[0099] Condition (1) and (1') specify the shape factor of the positive
single lens element 14 when the second sub-lens group G1b (i.e., the
focusing lens group) is configured of the positive single lens element
14. By forming the positive single lens element 14 so as to have a
positive meniscus shape, having a convex surface on the object side, that
satisfies condition (1) or (1'), abaxial astigmatism at the short focal
length extremity can be favorably corrected; coma, astigmatism and field
curvature during a focusing operation can be favorably corrected; and a
favorable balance of aberration fluctuations over the entire zoom lens
system can be maintained when a focusing operation is carried out.

[0100] If the upper limit of condition (1) is exceeded, the difference
between the radius of curvature of the surface on the object side of the
positive single lens element 14 and the radius of curvature of the
surface on the image side of the positive single lens element 14 becomes
substantially zero (0), so that it becomes difficult to suppress coma
fluctuations during a focusing operation.

[0101] If the lower limit of condition (1) is exceeded, the positive
single lens element 14, which constitutes the focusing lens group,
becomes a planoconvex positive lens element having a convex surface on
the object side, and it becomes difficult to suppress the astigmatism
fluctuations during a focusing operation. Moreover, correction of the
field curvature becomes insufficient.

[0102] Condition (2) specifies the ratio of the focal length of the second
sub-lens group G1b (focusing lens group, i.e., the positive single lens
element 14) to the focal length of the first sub-lens group G1a. By
satisfying condition (2), various aberrations such as spherical
aberration, coma, distortion and astigmatism can be favorably corrected.

[0103] If the upper limit of condition (2) is exceeded, the refractive
power of the second sub-lens group G1b (the focusing lens group) becomes
too strong, which although has the advantage of reducing the amount of
movement of the second sub-lens group G1b toward the close-distance side,
it becomes difficult to correct spherical aberration and coma at the long
focal length extremity.

[0104] If the lower limit of condition (2) is exceeded, the refractive
power of the first sub-lens group G1a becomes too strong, so that it
becomes difficult to correct distortion and astigmatism.

[0105] Condition (3) specifies the balance of refractive power between the
three negative lens elements 11, 12 and 13 of the first sub-lens group
G1a. By satisfying condition (3), the entire zoom lens system can be made
more compact (miniaturized) by reducing the diameter of the first lens
group G1, the cost of the glass lens material can be reduced, and
astigmatism and distortion at the short focal length extremity can be
favorably corrected.

[0106] If the upper limit of condition (3) is exceeded, the refractive
power of the negative lens element 11 within the first sub-lens group G1a
becomes too strong, so that the radius of curvature of the negative lens
element 11 is decreased, thereby increasing the cost of the glass
material thereof. Moreover, correction of astigmatism at the short focal
length extremity becomes difficult.

[0107] If the lower limit of condition (3) is exceeded, the refractive
power of the negative lens element 11 within the first sub-lens group G1a
becomes too weak, so that the entire zoom lens system is enlarged due to
the diameter of the first lens group G1 increasing in size. Moreover,
correction of distortion at the short focal length extremity becomes
difficult.

[0108] Specific first through ninth numerical embodiments of the zoom lens
system according to the present invention will be herein discussed. In
the various aberration diagrams, lateral aberration diagrams and the
tables, the d-line, g-line and C-line show aberrations at their
respective wave-lengths; S designates the sagittal image, M designates
the meridional image, FNO. designates the f-number, f designates the
focal length of the entire optical system, W designates the half angle of
view (°), Y designates the image height, fB designates the
backfocus, L designates the overall length of the lens system, r
designates the radius of curvature, d designates the lens thickness or
distance between lenses, N(d) designates the refractive index at the
d-line, and νd designates the Abbe number with respect to the d-line.
The unit used for the various lengths is defined in millimeters (mm). The
values for the f-number, the focal length, the half angle-of-view, the
image height, the backfocus, the overall length of the lens system, and
the distance between lenses (which changes during zooming) are shown in
the following order: short focal length extremity, intermediate focal
length, and long focal length extremity.

[0109] An aspherical surface which is rotationally symmetrical about the
optical axis is defined as:

[0111] FIGS. 1 through 6D and Tables 1 through 4 show a first numerical
embodiment of a zoom lens system according to the present invention. FIG.
1 shows a lens arrangement of the first numerical embodiment of the zoom
lens system at the long focal length extremity when focused on an object
at infinity. FIGS. 2A, 2B, 2C and 2D show various aberrations that
occurred in the lens arrangement shown in FIG. 1. FIGS. 3A, 3B, 3C and 3D
show lateral aberrations that occurred in the lens arrangement shown in
FIG. 1. FIG. 4 shows a lens arrangement of the first numerical embodiment
of the zoom lens system at the short focal length extremity when focused
on an object at infinity. FIGS. 5A, 5B, 5C and 5D show various
aberrations that occurred in the lens arrangement shown in FIG. 4. FIGS.
6A, 6B, 6C and 6D show lateral aberrations that occurred in the lens
arrangement shown in FIG. 4. Table 1 shows the lens surface data, Table 2
shows various data of the zoom lens system, Table 3 shows the aspherical
surface data, and Table 4 shows various data of the lens groups according
to the first numerical embodiment of the present invention.

[0112] The zoom lens system of the present invention is configured of a
negative first lens group G1 and a positive second lens group G2, in that
order from the object side.

[0113] The first lens group G1 is configured of a negative first sub-lens
group G1a and a positive second sub-lens group G1b, in that order from
the object side.

[0114] The first sub-lens group G1a is configured of a negative meniscus
lens element 11 having a convex surface on the object side, a negative
meniscus lens element 12 having a convex surface on the object side, and
a negative meniscus lens element 13 having a convex surface on the object
side, in that order from the object side. The negative meniscus lens
element 11 that is provided closest to the object side within the first
sub-lens group G1a is a hybrid lens configured of an aspherical layer
formed from a compound resin material bonded onto a glass lens element.

[0115] The second sub-lens group G1b is configured of a positive meniscus
single lens element 14 having a convex surface on the object side. The
positive meniscus single lens element 14 (second sub-lens group G1b)
constitutes a focusing lens group which is moved in the optical axis
direction during a focusing operation. In other words, upon carrying out
a focusing operation so as to focus on an object at infinity through to
an object at a finite distance, the positive meniscus single lens element
14 (second sub-lens group G1b) is moved in the optical axis direction
toward the image side.

[0116] The second lens group G2 is configured of a biconvex positive lens
element 21, a cemented lens having a biconvex positive lens element 22
and a biconcave negative lens element 23; and a biconvex positive lens
element 24, in that order from the object side. Each of the biconvex
positive lens elements 21 and 24 is provided with an aspherical surface
on each side thereof. The diaphragm S which is provided in between the
second sub-lens group G1b (first lens group G1) and the second lens group
G2 moves integrally with the second lens group G2 in the optical axis
direction. An optical filter OP and a cover glass CG are positioned
behind the second lens group G2 (the biconvex positive lens element 24)
(in between the second lens group G2 and the imaging plane I).

[0117] FIGS. 7 through 12D and Tables 5 through 8 show a second numerical
embodiment of a zoom lens system according to the present invention. FIG.
7 shows a lens arrangement of the second numerical embodiment of the zoom
lens system at the long focal length extremity when focused on an object
at infinity. FIGS. 8A, 8B, 8C and 8D show various aberrations that
occurred in the lens arrangement shown in FIG. 7. FIGS. 9A, 9B, 9C and 9D
show lateral aberrations that occurred in the lens arrangement shown in
FIG. 7. FIG. 10 shows a lens arrangement of the second numerical
embodiment of the zoom lens system at the short focal length extremity
when focused on an object at infinity. FIGS. 11A, 11B, 11C and 11D show
various aberrations that occurred in the lens arrangement shown in FIG.
10. FIGS. 12A, 12B, 12C and 12D show lateral aberrations that occurred in
the lens arrangement shown in FIG. 10. Table 5 shows the lens surface
data, Table 6 shows various data of the zoom lens system, Table 7 shows
the aspherical surface data, and Table 8 shows various data of the lens
groups according to the second numerical embodiment of the present
invention.

[0118] The lens arrangement of the second numerical embodiment is the same
as that of the first numerical embodiment.

[0119] FIGS. 13 through 18D and Tables 9 through 12 show a third numerical
embodiment of a zoom lens system according to the present invention. FIG.
13 shows a lens arrangement of the third numerical embodiment of the zoom
lens system at the long focal length extremity when focused on an object
at infinity. FIGS. 14A, 14B, 14C and 14D show various aberrations that
occurred in the lens arrangement shown in FIG. 13. FIGS. 15A, 15B, 15C
and 15D show lateral aberrations that occurred in the lens arrangement
shown in FIG. 13. FIG. 16 shows a lens arrangement of the third numerical
embodiment of the zoom lens system at the short focal length extremity
when focused on an object at infinity. FIGS. 17A, 17B, 17C and 17D show
various aberrations that occurred in the lens arrangement shown in FIG.
16. FIGS. 18A, 18B, 18C and 18D show lateral aberrations that occurred in
the lens arrangement shown in FIG. 16. Table 9 shows the lens surface
data, Table 10 shows various data of the zoom lens system, Table 11 shows
the aspherical surface data, and Table 12 shows various data of the lens
groups according to the third numerical embodiment of the present
invention.

[0120] The lens arrangement of the third numerical embodiment is the same
as that of the first numerical embodiment except that the negative lens
element 13 of the first sub-lens group G1a is a biconcave negative lens
element.

[0121] FIGS. 19 through 24D and Tables 13 through 16 show a fourth
numerical embodiment of a zoom lens system according to the present
invention. FIG. 19 shows a lens arrangement of the fourth numerical
embodiment of the zoom lens system at the long focal length extremity
when focused on an object at infinity. FIGS. 20A, 20B, 20C and 20D show
various aberrations that occurred in the lens arrangement shown in FIG.
19. FIGS. 21A, 21B, 21C and 21D show lateral aberrations that occurred in
the lens arrangement shown in FIG. 19. FIG. 22 shows a lens arrangement
of the fourth numerical embodiment of the zoom lens system at the short
focal length extremity when focused on an object at infinity. FIGS. 23A,
23B, 23C and 23D show various aberrations that occurred in the lens
arrangement shown in FIG. 22. FIGS. 24A, 24B, 24C and 24D show lateral
aberrations that occurred in the lens arrangement shown in FIG. 22. Table
13 shows the lens surface data, Table 14 shows various data of the zoom
lens system, Table 15 shows the aspherical surface data, and Table 16
shows various data of the lens groups according to the fourth numerical
embodiment of the present invention.

[0122] The lens arrangement of the fourth numerical embodiment is the same
as that of the first numerical embodiment.

[0123] FIGS. 25 through 30D and Tables 17 through 20 show a fifth
numerical embodiment of a zoom lens system according to the present
invention. FIG. 25 shows a lens arrangement of the fifth numerical
embodiment of the zoom lens system at the long focal length extremity
when focused on an object at infinity. FIGS. 26A, 26B, 26C and 26D show
various aberrations that occurred in the lens arrangement shown in FIG.
25. FIGS. 27A, 27B, 27C and 27D show lateral aberrations that occurred in
the lens arrangement shown in FIG. 25. FIG. 28 shows a lens arrangement
of the fifth numerical embodiment of the zoom lens system at the short
focal length extremity when focused on an object at infinity. FIGS. 29A,
29B, 29C and 29D show various aberrations that occurred in the lens
arrangement shown in FIG. 28. FIGS. 30A, 30B, 30C and 30D show lateral
aberrations that occurred in the lens arrangement shown in FIG. 28. Table
17 shows the lens surface data, Table 18 shows various data of the zoom
lens system, Table 19 shows the aspherical surface data, and Table 20
shows various data of the lens groups according to the fifth numerical
embodiment of the present invention.

[0124] The lens arrangement of the fifth numerical embodiment is the same
as that of the third numerical embodiment.

[0125] FIGS. 31 through 36D and Tables 21 through 24 show a sixth
numerical embodiment of a zoom lens system according to the present
invention. FIG. 31 shows a lens arrangement of the sixth numerical
embodiment of the zoom lens system at the long focal length extremity
when focused on an object at infinity. FIGS. 32A, 32B, 32C and 32D show
various aberrations that occurred in the lens arrangement shown in FIG.
31. FIGS. 33A, 33B, 33C and 33D show lateral aberrations that occurred in
the lens arrangement shown in FIG. 31. FIG. 34 shows a lens arrangement
of the sixth numerical embodiment of the zoom lens system at the short
focal length extremity when focused on an object at infinity. FIGS. 35A,
35B, 35C and 35D show various aberrations that occurred in the lens
arrangement shown in FIG. 31. FIGS. 36A, 36B, 36C and 36D show lateral
aberrations that occurred in the lens arrangement shown in FIG. 34. Table
21 shows the lens surface data, Table 22 shows various data of the zoom
lens system, Table 23 shows the aspherical surface data, and Table 24
shows various data of the lens groups according to the sixth numerical
embodiment of the present invention.

[0126] The lens arrangement of the sixth numerical embodiment is the same
as that of the third numerical embodiment.

[0127] FIGS. 37 through 42D and Tables 25 through 28 show a seventh
numerical embodiment of a zoom lens system according to the present
invention. FIG. 37 shows a lens arrangement of the seventh numerical
embodiment of the zoom lens system at the long focal length extremity
when focused on an object at infinity. FIGS. 38A, 38B, 38C and 38D show
various aberrations that occurred in the lens arrangement shown in FIG.
37. FIGS. 39A, 39B, 39C and 39D show lateral aberrations that occurred in
the lens arrangement shown in FIG. 37. FIG. 40 shows a lens arrangement
of the seventh numerical embodiment of the zoom lens system at the short
focal length extremity when focused on an object at infinity. FIGS. 41A,
41B, 41C and 41D show various aberrations that occurred in the lens
arrangement shown in FIG. 40. FIGS. 42A, 42B, 42C and 42D show lateral
aberrations that occurred in the lens arrangement shown in FIG. 40. Table
25 shows the lens surface data, Table 26 shows various data of the zoom
lens system, Table 27 shows the aspherical surface data, and Table 28
shows various data of the lens groups according to the seventh numerical
embodiment of the present invention.

[0128] The lens arrangement of the seventh numerical embodiment is the
same as that of the first numerical embodiment except for the following
configurations (1) through (3):

[0129] (1) The negative meniscus lens element 11 of the first sub-lens
group G1a is a spherical-surfaced lens element (i.e., is not a hybrid
lens).

[0130] (2) The negative lens element 12 of the first sub-lens group G1a
has an aspherical surface on each side thereof.

[0131] (3) The second lens group G2 is configured of a positive meniscus
lens element 21' having a convex surface on the object side, a biconvex
positive lens element 22', a cemented lens having a biconvex positive
lens element 23' and a biconcave negative lens element 24'; and a
biconvex positive lens element 25', in that order from the object side.
The biconvex positive lens element 22' and the biconvex positive lens
element 25' each have an aspherical surface on each side thereof.

[0132] FIGS. 43 through 48D and Tables 29 through 32 show an eighth
numerical embodiment of a zoom lens system according to the present
invention. FIG. 43 shows a lens arrangement of the eighth numerical
embodiment of the zoom lens system at the long focal length extremity
when focused on an object at infinity. FIGS. 44A, 443, 44C and 44D show
various aberrations that occurred in the lens arrangement shown in FIG.
43. FIGS. 45A, 45B, 45C and 45D show lateral aberrations that occurred in
the lens arrangement shown in FIG. 43. FIG. 46 shows a lens arrangement
of the eighth numerical embodiment of the zoom lens system at the short
focal length extremity when focused on an object at infinity. FIGS. 47A,
47B, 47C and 47D show various aberrations that occurred in the lens
arrangement shown in FIG. 46. FIGS. 48A, 48B, 48C and 48D show lateral
aberrations that occurred in the lens arrangement shown in FIG. 46. Table
29 shows the lens surface data, Table 30 shows various data of the zoom
lens system, Table 31 shows the aspherical surface data, and Table 32
shows various data of the lens groups according to the eighth numerical
embodiment of the present invention.

[0133] The fundamental lens arrangement of the eighth numerical embodiment
is the same as that of the third numerical embodiment.

[0134] FIGS. 49 through 54D and Tables 33 through 36 show a ninth
numerical embodiment of a zoom lens system according to the present
invention. FIG. 49 shows a lens arrangement of the ninth numerical
embodiment of the zoom lens system at the long focal length extremity
when focused on an object at infinity. FIGS. 50A, 50B, 50C and 50D show
various aberrations that occurred in the lens arrangement shown in FIG.
49. FIGS. 51A, 51B, 51C and 51D show lateral aberrations that occurred in
the lens arrangement shown in FIG. 49. FIG. 52 shows a lens arrangement
of the ninth numerical embodiment of the zoom lens system at the short
focal length extremity when focused on an object at infinity. 53A, 53B,
53C and 53D show various aberrations that occurred in the lens
arrangement shown in FIG. 52. FIGS. 54A, 54B, 54C and 54D show lateral
aberrations that occurred in the lens arrangement shown in FIG. 52. Table
33 shows the lens surface data, Table 34 shows various data of the zoom
lens system, Table 35 shows the aspherical surface data, and Table 36
shows various data of the lens groups according to the ninth numerical
embodiment of the present invention.

[0135] The lens arrangement of the ninth numerical embodiment is the same
as that of the first numerical embodiment except that the second lens
group G2 is configured of a positive meniscus lens element 21' having a
convex surface on the object side, a biconvex positive lens element 22',
a cemented lens having a biconvex positive lens element 23' and a
biconcave negative lens element 24'; and a biconvex positive lens element
25', in that order from the object side. The biconvex positive lens
element 22' and the biconvex positive lens element 25' each have an
aspherical surface on each side thereof.

[0137] As can be understood from Table 37, the first through ninth
embodiments satisfy conditions (1) through (3). Furthermore, as can be
understood from the aberration diagrams, the various aberrations are
suitably corrected.

[0138] Obvious changes may be made in the specific embodiments of the
present invention described herein, such modifications being within the
spirit and scope of the invention claimed. It is indicated that all
matter contained herein is illustrative and does not limit the scope of
the present invention.